This invention pertains to noise reduction in fiber optic interferometric systems used for sensing. More particularly, it pertains to such noise reduction in hydrophone sensor systems employing pairs of Michelson or Mach-Zehnder interferometers at each spatial sensing point. The apparatus and methods apply as well to measurement of field quantities such as sound, pressure and temperature.
Both Michelson and Mach-Zehnder interferometers are used to measure acoustic pressure and temperature. Sound is measured by detecting the instantaneous fluid pressure. The arms of the interferometer are immersed in the fluid whose pressure, temperature, or other parameter is to be measured. The measured parameter will be recited herein as either pressure or acoustic pressure with the understanding that the invention is not limited to pressure or acoustic pressure.
Usually one arm of the interferometer is sensitized to changes in fluid pressure, whereas the other is not. Alternatively, one arm may extend and the other contract with increases in pressure. Frequently, to reduce noise, the output signal of an identical interferometer, whose arms are not exposed to the sensed pressure, is subtracted from the sensor interferometer output signal.
It is found that, because the coherence length of the driving laser is finite, phase errors or phase noise are produced in the output signal.
In the prior art, both Mach-Zehnder and Michelson interferometric sensors have only one input port and one output port. Single-mode directional couplers are used to split and recombine the light beams. In a typical operation, one of the fiber arms is sensitive to the field quantity to be measured, and the other arm is isolated. A change in pressure at the sensor causes a differential change in phase between the two recombined beams, producing an intensity change at the detector.
In a frequency-division-multiplexed system, FM-modulated coherent light, combined with a small delay-time imbalance between the interferometer fiber paths produces a phase-modulated carrier at the detector, simplifying signal demodulation. Intensity noise is also generated by the combination of laser phase noise and the delay time imbalance. Both the desired sensor signal and the laser phase-noise appear in the sensor output signal. Prior-art interferometric sensors employing FM-modulated sources and slightly-unbalanced paths are limited in performance by the phase-noise of the laser.
In other prior art embodiments, both available output signals from Mach-Zehnder interferometric sensors are used. Changes in the intensity of such sensor signals are differential. The phases of the sensed signals and the phase of the laser noises are not common-mode. Differencing the two output signals increases the signal-to-noise ratio 3dB, for the carrier power increases 6dB, but the noise increases only 3 dB.
Single-output Frequency-Division-Multiplexed (FDM) interferometric sensors are limited by phase noise from the combined effect of finite laser linewidth and the slight delay-time imbalance in the sensor.
One prior art apparatus that purports to eliminate or reduce laser phase noise uses a remote isolated interferometer whose delay-time difference is matched to that of the sensor. Following photodetection and demodulation, the signals from the isolated interferometer and from the sensor interferometer are electronically differenced, cancelling phase noise. The sensed signal is unchanged because the remote isolated reference interferometer is not exposed to the physical stimulus which is sensed by the sensors. Only one reference interferometer is needed for a plurality of sensors, all driven by a common laser source. The demodulated electrical output signals from the reference interferometer contains only laser phase noise. That output is subtracted from the outputs of all of the sensor channels, cancelling the laser phase noise while preserving the desired signal.
Such apparatus has achieved limited success in the laboratory, but it is believed to have limited potential in the field. Separate mechanical packages are needed for the sensor and the reference, and the reference must be well-isolated from the environment. Second order non-linear terms are not eliminated. Noise is also produced by the telemetry connecting the laser to the sensor. Further, failure to accurately match the delay-time difference between any sensor and the reference interferometer reduces noise cancellation. The matching condition is often difficult to maintain over widely-varying environmental conditions.